PASCO OS-8501 Interferometer - Manual (Page 12)

scientific
Interferometer 012-02675B
8
Theory
For light of a specific frequency, the wavelength
λ
varies according to the formula:
λ
=
λ
o
/n;
where
λ
o
is the wavelength of the light in a vacuum, and n is the index of refraction for the
material in which the light is propagating. In this experiment, you will use the interferometer
to measure the index of refraction for air.
For reasonably low pressures, the
index of refraction for a gas varies
linearly with the gas pressure. Of
course for a vacuum, where the
pressure is zero, the index of
refraction is exactly 1. A graph for
the refraction index versus gas
pressure is shown in Figure 8.
The measurements you make in
this experiment will allow you to
calculate the slope of this graph for
air. From that, numerical values
can be determined for the index of
air refraction at various pressures.
Procedure
➀
Align the laser and interferometer as described earlier in this manual.
➁
The experimental set-up is shown in Figure 9. Push the air hose of the vacuum pump over the
air outlet hole of the vacuum chamber. Then plug the banana plug of the vacuum chamber into
the hole in the interferometer base – between the fixed mirror and the beam-splitter.
➂
Adjust the alignment screws of the fixed mirror so the center of the interference pattern is
clearly visible on the viewing screen. (The fringe pattern will be somewhat distorted by
irregularities in the glass end-plates of the vacuum chamber.)
Notice that the banana plug of the vacuum chamber is free to rotate in its hole. For accurate
measurements, the end-plates of the vacuum chamber must be perpendicular to the laser beam.
Rotate the chamber slowly and observe the effect on the interference fringes. Based on your
observations, how can you be sure that the end-plates of the vacuum chamber are properly
aligned?
➄ Be sure that the air in the vacuum chamber is at atmospheric pressure. If you are using a
PASCO Hand Vacuum Pump, this is performed by flipping the vacuum release toggle switch.
(Alternatively, many people find it more convenient to begin with the vacuum chamber
evacuated – they then let air into the chamber as the fringes are counted. Use whichever
Exp 2: Measuring the Index of Refraction for Air
Figure 8 GRAPH,
INDEX OF REFRACTION VS PRESSURE
INDEX OF
REFRECTION
(n)
2
1
0
0
GAS PRESSURE (cm Hg)
➃